Organic light-emitting diode (OLED) display is also called organic electroluminescent display, which differs in the way of display from traditional liquid crystal display (LCD). An OLED does not need a backlight and is formed with a very thin light-emitting layer and a substrate. Therefore, an OLED display device can be made lighter, thinner, with wider viewable angle, and significantly save electric energy. Therefore, the OLED display device is becoming more and more popular.
FIG. 1 is the schematic diagram of the electroluminescent element in prior art. As shown in FIG. 1, the electroluminescent element in prior art comprises a substrate 101, and an anode layer 102, a light-emitting layer, and a cathode layer 104 that are formed in sequence on the substrate 101, with the anode layer 102 and the cathode layer 104 being respectively connected to the positive and negative electrodes of a power supply. The substrate 101 may be a glass substrate, a polyester substrate, or a polyimide compound substrate, etc.; the anode layer 102 can employ an inorganic metal oxide, such as indium tin oxide (ITO), zinc oxide (ZnO), etc., as well as a high work function metal material, such as gold, copper, silver, platinum, etc.; the cathode layer 104 can employ a low work function metal material, such as lithium, magnesium, calcium, strontium, aluminum, indium, etc.; and the material for the light-emitting layer can include a fluorescent material, such as NPB (N,N′-diphenyl-N,N′-bis(1-naphthyl-phenyl)-1,1′-biphenyl-4,4′-diamine), DPVBI (4,4′-bis(2,2-disphenylvinyl)biphenyl), etc.
In prior art, researchers primarily focused on obtaining high efficiency organic electroluminescent element by exploiting triplet excitons and singlet excitons. However, since the transport of carriers is not ideal, carriers may be injected under a unbalance state, the injected carriers may not be recombined a hundred percent for luminescence, and the excitons formed by the recombination of carriers may even be quenched, which all causes the actual efficiency far lower than the maximal theoretical value. Meanwhile, when a voltage is applied to the light-emitting layer 103 via the anode layer 102 and the cathode layer 104, the hole mobility is higher than the electron mobility. However, with the increase of voltage, the increase of the electron mobility will be faster than the increase of the mobility of the hole mobility, so the position of the recombination zone in the light-emitting layer will change along with the change of the voltage, resulting in notable change of the color of the element along with the change of the voltage. If elements doped with P and N dopants, as well as the light-emitting layer is formed using a matrix material favoring electron transport and there is only a single recombination zone, then the light-emitting layer will be closer to the hole transport layer. This will also cause extra excitons spread to the hole transport layer, resulting in non-radiative decay and the quantity mismatch between the carriers such as electrons and holes in the light-emitting layer 103 and reducing the light-emitting efficiency of the light-emitting layer 103, which is detrimental to the improvement of the efficiency of the element.